The interaction between irreversibly heparinized materials and blood is being studied. Particular attention is being directed to the oft-stated, yet unproven, limitations to the long term use of heparin in biomaterials: heparin elution is necessary for thrombo-resistance, saturation of the surface with inactive complex occurs, clotting factors are consumed and platelet deposition is enhanced. Careful determination of the elution rate of heparin from the heparin-PVA hydrogel beads under study indicate that the elution rate (approximately 10 to the minus 11th power gm/sq.cm.min.) is much less than that needed for thromboresistaace of ionically heparinized materials. Nevertheless, the plasma recalcification time is prolonged and thrombin is inactivated by antithrombin III in the presence of these beads. Detailed investigation of the thrombin inactivation reaction on heparin-PVA beads suggests that thrombin-heparin interaction is the primary step in enzyme inactivation and that thrombin may be activated by contact with the surface bound heparin. Future plans include the study of the mechanism of the heparin-PVA reaction and the exchange between bound and unbound thrombin and antithrombin III. In this way, the surface saturation limitation to the long term use of heparin can be assessed.